Electric cables of reduced micro-voids in the extruded insulation

ABSTRACT

A product by process for reducing micro-voids in cross-linked cable insulation without reducing the voltage breakdown strength thereof in which process the insulation, after cross-linking, is heated at a temperature in the range from about 120° C. to about 150° C. and under a pressure less than 10 mm. Hg for a period from about 8 hours to 48 hours. Also, a cable having such insulation which has micro-voids less than about 10 2  per mm 3  and a perforation gradient greater than the perforation gradient of a cable which has not been so treated.

This is a division of application Ser. No. 30,726, filed 4/17/79 nowU.S. Pat. No. 4,259,281.

The present invention refers to a cable process for reducing micro-voidswhich form in the extruded insulation of medium and high voltageelectric cables, as a consequence of the step of cross-linking of theinsulation itself.

Extruded insulation means herein a plastic material, such aspolyethylene or other polyolefinic compound or an elastomeric material,which is extruded around the conductor or conductors of an electriccable and which is afterwards cross-linked.

As is known, the cross-linking process (or vulcanization) has the objectof increasing the temperature range in which the mechanical strength ofthe insulation is sufficient to permit the use of the insulation. Saidprocess essentially consists of a high temperature treatment of theinsulation in the presence of substances which decompose, at a suchtemperature, into radicals which link to the macromolecules and formingtransverse bridges among the polymeric chains. The substances generallyused as cross-linking agents are organic peroxides.

The cross-linking processing of the insulation can be technicallycarried out in different ways, each of which has advantages anddrawbacks. The more usual process (both continuous and discontinuous)consists of heating the insulation by saturated steam at high pressure.The working conditions are about 200° C. of temperature and 14atmospheres of relative pressure, and the time to complete thecross-linking depends essentially on the nature and the thickness of theextruded insulation. The main advantages of this method are of thesimplicity of the processing plant and the low operating cost, whereasthe drawback is that of a great diffusion of the steam into the plasticmass.

During the following cooling process, which usually is quite rapid, onlya small part of the water, which was dissolved in the mass, is able todiffuse to the outside thereof, and the other part of the water remainsin the consolidated mass, generally, in the form of microscopic drops.Even if the water is able to leave the mass slowly, the space taken upby said microscopic drops does not close up immediately. As a result, inthe solid mass of the insulation, there is a great quantity of verysmall cavities (micro-voids) partly filled with water and partly empty.

The order of magnitude of the diameter of these cavities is of fewmicrons, i.e. 0.1 to 10 microns. However, said cavities can sometimes beso numerous and so near the one to the other as to appear as opalescentzones or "milky-zones". An approximate quantitative determination of thenumbered micro-voids per unit volume can be made by examining insulationsections under a microscope.

Actually, not only the steam, but also the decomposition products of thecross-linking agent contribute to the formation of these micro-voids,the cross-linking agent contributing by a mechanisms similar to thatdescribed for the steam. The contribution of said decomposition productsis, without doubt, great even if, from a quantitative point of view, thecontribution is clearly lower with respect to contribution of the steam.

It has been shown that both the steam and the decomposition products ofthe cross-linking agent succeed spontaneously, even if slowly, indiffusing towards the outside of the consolidated mass and that themicro-voids forming in consequence of the migration and release of saidproducts close again with the passing of time, by virtue of thespontaneous and random, even if small, movements of the macromolecules.However, this spontaneous closing process of the micro-voids can lastyears, so that, as a matter of fact, said micro-voids remain in the massof the insulation for a very long time.

It is not yet completely clear how much these micro-voids affect thedielectric properties of the extruded insulation which surrounds thecable conductors, but certainly, the dielectric properties of theinsulation are reduced in a way not determinable in advance andforseeable. In order to avoid this hazardous drawback, cross-linkingprocesses alternative to the process of using saturated steam havealready been proposed and carried out for several years. Said processesare generically known to those skilled in the art as dry cross-linking,or dry-curing processes. Included in such known techniques are theprocesses which use, as means for heat transmission, one of thefollowing fluids under pressure: inert gas, glycola of high molecularweight, melted salt baths, low-melting alloys, e.g. Wood's metal,silicone oils, etc. In other dry-curing processes, the cross-linking iscaused by heat radiation or by ultra-sonic energy.

In another process, resort is had to the expedient of covering, beforethe cross-linking by steam, the extruded insulation with thin sheaths ofplastic material containing moisture acceptor compounds, for example,calcium oxide which prevent or reduce the diffusion of the steam intothe extruded mass in contact with the conductors.

In effect, all the aforementioned dry, cross-linking processes permit areduction in the number of micro-voids, but not beyond a certain limit,since said processes cannot prevent the formation of micro-voids causedby the decomposition products of the cross-linking agent. Moreover, allthese dry-curing processes require complicated plants and rathercritical working conditions. Also the economic burden, including thecost of the raw materials and of the energy consumption, is clearlyhigher with respect to the process of cross-linking using saturatedsteam.

Therefore, it is desirable to make use of all the technological andeconomic advantages offered by the cross-linking process using saturatedsteam by causing the elimination of the small water drops and of thedecomposition products of the cross-linking agent by means of atreatment after the cross-linking process is completed.

From a theoretical point of view, it appears that such a treatment ofthe cross-linked insulation, consisting practically of degassing a masssuch as that in question, would have to be based on a heat treatment ata reduced pressure. The effectiveness of such treatment in degassing isconfirmed by the results reported in the publication entitled"Micro-voids in Cross-Linked Polyethylene Insulated Cables," IEEETransactions on Power Apparatus and System Review (vol. PAS-94, n.4,July/August 1975, pages 1258-1263). In the said publication, however, noindication of the working conditions (temperature, pressure, time) ofthe treatment is given. Therefore, it is not possible to get any actualtechnical teaching from this publication. The article provides onlyexperimental confirmation of the theory that heating at a reducedpressure of a plastic mass favours the removal of the volatilesubstances included in it and the consequent closing of the micro-voids,in consequence of the increased movement intensity of the macromoleculeson the walls of the micro-voids as a result of the heat.

Moreover, from the data reported in table 4 of said publication, one isled to believe that a heat treatment at a low pressure after thecross-linking process, degrades at least partially, the dielectricproperties of the extruded insulating material. In fact, with regard tothe examples 1 and 2 therein, which refer to the more usually usedmaterial, i.e. the cross-linked polyethylene, it is noted that thenominal perforation gradient with alternate current (BDV) undergoes, inconsequence of said treatment, a significant reduction.

Therefore, said publication, on the one hand does not report anyoperating condition with respect to the elimination of the micro-voids,and on the other hand does not consider that the heat treatment at lowpressure is favourable with respect to an improvement of the dielectricproperties of the extruded insulation.

On the contrary, the applicants have found, to their surprise, that itis possible to establish ranges of values for the temperature, thepressure and the time, in which it is possible to obtain an optimumresult with respect to the reduction of micro-voids and without damageto the dielectric properties of the insulation, which properties are, onthe contrary significantly improved.

One object of the present invention is to provide working conditions forthe treatment of cross-linked insulation to improve such insulationwhich conditions do not refer only to the extruded insulation material,but also refer to the manufactured article comprising the metallicconductor and the extruded insulation. As is known, this involves thediscovery of new elements which are not always foreseeable either on atheoretical basis or on an experimental one.

Therefore, the present invention comprises a teaching contrary to theteachings of the prior art and provides, moreover, working paramatersadvantageously applicable on an industrial scale.

Accordingly, one object of the present invention is a process forreducing micro-voids in the extruded insulation around the conductor orconductors of a medium and high voltage electric cable, characterized bythe fact that it consists of a heat treatment at a low pressure afterthe cross-linking process of said extruded insulation in the presence ofcross-linking agents, the temperature of said heat treatment beingbetween about 120° and about 150° C. and the pressure of the insulationambient being lower than 10 mm. Hg. The time required for said heattreatment is, under static condition, between about 8 and about 48hours.

A further object of the present invention is an electric cable suitablefor use at medium and high voltages, which is made by subjecting theextruded insulation, after the cross-linking, to a treatment forreducing the micro-voids according to the present invention.

Other objects and advantages of the invention will be apparent from thefollowing detailed description of preferred embodiments thereof, whichdescription should be considered in conjunction with the accompanyingdrawing, the single figure of which is a cross-section of a cable madein accordance with the invention.

The cable core 1 illustrated in FIG. 1 has a central conductor 2, asemi-conducting layer 3 around the conductor 2, a cross-linked layer 4of insulation treated in accordance with the invention and asemi-conducting layer 5. The final cable may, of course, include otherlayers, such as an outer protective sheath, and the process of theinvention is applicable to cables including a plurality of insulatedconductors.

According to a first example of the invention, a red copper conductor 2having a section of 240 mm² able to operate at a rated voltage of 60 kV,is covered with a first semi-conducting layer 3, with an insulatinglayer 4 of polyethylene to be cross-linked and having a thickness of 14mm, and lastly with a second semi-conductor layer 5.

The polyethylene contains, as the cross-linking agent, dicumylperoxidein a quantity of 1.8 parts by weight per 100 parts by weight ofpolyethylene. The semi-conductor layers are made of ethylene/propylenerubber loaded with conductive carbon black.

The so-obtained core 1, formed by the conductor 2, the extrudedinsulation 4 and the semi-conducting layers 3 and 5 is moved along acontinuous cross-linking line. Said cross-linking line is constituted bya tube in which the advancing core is covered, in counter-current, bysaturated steam at a temperature of about 200° C. and therefore, at apressure of about 14 atmospheres above atmospheric pressure.

At the end of the cross-linking process, the insulation contains aquantity of micro-voids, of a diameter comprised between 0.1-10 microns,equal to 10⁴ -10⁵ per mm³. It has been established that the nominalvoltage breakdown strength or perforation gradient of such insulationfor alternating current for cable lengths of 10 meters is 39 kV/mm.

The core 1 coming out from the cross-linking line is wound up in coils,which are subsequently disposed in a heated chamber and at low pressurefor the treatment for the reduction of micro-voids according to thepresent invention.

In order to make the heating of the core 1 easier and, therefore, toreduce the treatment times, it is convenient to take advantage of theJoule effect, that is, to connect the ends of the conductor 2 to acurrent source and cause current to circulate in the conductor 2. Theheating of the core 1 at the conductor 2 has also the great advantage ofdirecting the diffusion and the ejection of the contained volatilesubstance, steam and degradation products of the cross-linking agent,toward the outer surfaces of the wound coil.

The preferred treatment conditions, at steady state, are as follows:135° C. for a median temperature of the conductor 2 carrying current,130° C. for the median temperature of the chamber, 1 mm. of mercury forthe pressure and 24 hours for the time.

At the end of said treatment, the extruded insulation shows under amicroscope the presence of few micro-voids, less than 10² per mm³,whereas the nominal perforation gradient for alternating current forcable lengths of 10 m. has increased to 52 kV/mm.

In a second example, a tinned copper conductor 2, having a section of630 mm² and designed to operate at a rated voltage of 60 kV, is covered,with successive extrusions, with a first semi-conductor layer 3, aninsulating layer 4 of polyethylene to be cross-linked having a thicknessof 13 mm, and, at last, with a second semi-conductor layer 5. Thepolyethylene contains, as the cross-linking agent, dicumylperoxide in aquantity of 1.8 parts by weight per 100 parts by weight of polyethylene.The semi-conductor layers are made of ethylene/propylene rubber loadedwith conductive carbon black.

The so-obtained core 1 is subjected to a cross-linking process withsaturated steam at high pressure under the same working conditions asthose described in the previous example. At the end of the cross-linkingprocess, the insulation 4 contains a quantity of micro-voids, of adiameter from 0.1 to 10 microns, equal to 10⁴ -10⁵ per mm³. It has beenestablished that the nominal performation gradient for such core withalternating current for cable lengths of 10 m is 37 kV/mm.

The coils of the wound core are thereafter subjected to the heattreatment according to the present invention at the same conditionsdescribed in the previous example.

The working conditions, at steady state, are as follows: 135° C. for amedian temperature of the conductor 2 carrying current, 130° C. for amedian chamber temperature, 1 mm. of mercury for pressure and 48 hoursfor the time.

At the end of said treatment the extruded insulation has fewmicro-voids, less than 10² per mm³, whereas the nominal perforationgradient for alternating current for cable lengths of 10 m. hasincreased to 50 kV/mm.

The following tables summarizes the working conditions of thecross-linking process by saturated steam and of the subsequent treatmentaccording to the present invention, and the dielectric properties of theextruded insulation at the end of the cross-linking and after thetreatment for reducing the micro-voids according to the presentinvention.

                  TABLE I                                                         ______________________________________                                                         Examples                                                                      No. 1    No. 2                                               ______________________________________                                        conductor    --        red copper tinned copper                               conductor    mm.sup.2  240        630                                         rated voltage                                                                              kV        60         60                                          insulating material                                                                        --        extruded   extruded                                                           polyethylene                                                                             polyethylene                                insulating thickness                                                                       mm        14         13                                          cross-linking agent                                                                        --        dicumylper-                                                                              dicumylper-                                                        oxide      oxide                                       cross-linking agent                                                                        pts. by wt.                                                                             1.8        1.8                                         of insulation                                                                 semiconductive         ethylene/  ethylene                                    layers                 propylene  propylene                                                          rubber loaded                                                                            rubber loaded                                                      with conduc-                                                                             with conduc-                                                       tive carbon                                                                              tive carbon                                                        black      black                                       ______________________________________                                    

                  TABLE II                                                        ______________________________________                                                             Example                                                                              Example                                                                1      2                                                 ______________________________________                                        CROSS-LINKING PROCESS                                                         saturated steam temperature                                                                    °C.                                                                              about 200                                                                              about 200                                 saturated steam pressure                                                                       relative                                                                      atm.      14       14                                        micro-voids in the insula-                                                                     micro-    10.sup.4 -10.sup.5                                                                     10.sup.4 -10.sup.5                        tion after the cross-                                                                          voids/                                                       linking          mm.sup.3                                                     nominal perforation                                                                            kV/mm     39       37                                        gradient with alternating                                                                      (for cable                                                   current after the cross-                                                                       lengths of                                                   linking          10 m.)                                                       TREATMENT FOR RE-                                                             DUCING MICRO-VOIDS                                                            median temperature of the                                                     conductor carrying current                                                                     °C.                                                                              135      135                                       median chamber temperature                                                                     °C.                                                                              130      130                                       pressure         mm Hg     1        1                                         treatment time   hours     24       48                                        micro-voids in the insula-                                                                     micro-    very rare                                                                              very rare                                 ting after the treatment                                                                       voids     less     less                                                       mm.sup.3  than 10.sup.2                                                                          than 10.sup.2                             nominal perforation gradient                                                                   kV/mm     52       50                                        with alternating current                                                                       (for cable                                                   after the treatment for re-                                                                    lengths of                                                   ducing micro-voids                                                                             10 m.)                                                       ______________________________________                                    

It will be observed from Tables I and II that the voltage breakdownstrength, or perforation gradient of the insulation increases by atleast 25% and that the number of micro-voids decreases to one-half orless. A cable treated in accordance with the invention may bedistinguished from an untreated same cable by having an increasedperforation gradient and not having in excess of 10³ micro-voids per mm³and preferably, less than 10² micro-voids per mm³.

Although, in the above two examples, the treatment for reducingmicro-voids is made after a cross-linking process by saturated steam athigh pressure, it will be apparent that the same treatment can beadvantageously applied also after other cross-linking processes,particularly those called dry cross-linking or dry-curing.

Although preferred embodiments of the present invention have beendescribed and illustrated, it will be apparent to those skilled in theart that various modifications may be made without departing from theprinciples of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An electric cable foruse at medium and high voltages, said cable comprising a conductorsurrounded by a first semi-conductive layer which is surrounded by solidinsulation which in turn is surrounded by second semi-conductive layer,said solid insulation being an extruded, peroxide cross-linked, plasticmaterial which has been treated subsequent to the cross-linking thereofby subjecting it to heat at a temperature in the range from about 120°C. to 150° C. and an ambient pressure less than 10 mm. Hg. to removedecomposition products and reduce the number of micro-voids formedduring the cross-linking, said insulation being characterized by aperforation gradient better than the same insulation which has not beenso treated, being substantially free of said decomposition products andhaving a number of micro-voids less than the same insulation which hasnot been so treated and less than 10³ per mm³ whereby the perforationgradient is improved and the stability of the insulating properties ofthe insulation remains substantially constant with use over a period oftime.
 2. An electric cable as set forth in claim 1 wherein theperforation gradient of the insulation is at least 10% greater than thesame cable untreated and the insulation has a number of micro-voids lessthan 10² per mm³.
 3. An electric cable as set forth in claim 1 whereinthe perforation gradient of the insulation is at least 45 kilovolts permm. and the insulation has a number of micro-voids less than 10² permm³.
 4. An electric cable as set forth in claim 3 wherein saidinsulation is unfilled, peroxide cross-linked polyethylene without avoltage stabilizer therein.